Relay valve combined with a load-sensing proportion valve

ABSTRACT

A combined load-sensing proportion and relay valve for an air brake system including a valve disk which has an exhaust passage in its axial center and is adapted to hold the valve in normally closed position, and a control piston unit which is actuated by an indicated pressure from a brake valve to move the valve disk and open the valve. The control piston unit includes a first pressure-sensing piston for controlling the valve-opening action of the valve disk and a second pressure-sensing piston coaxially connected to the first piston via a spring therebetween. A stroke adjusting device is provided to control the axial movement of the second piston correspondingly to the payload of the vehicle.

BACKGROUND OF THE INVENTION

This invention relates to a novel relay valve combined with aload-sensing proportion valve, and more specifically to a combinedload-sensing proportion and relay valve capable of automaticallyadjusting the pressure applicable to the power chambers of brakeactuators.

Rapid development of commercial transport by such large motor vehiclesas trailer-trucks in recent years has been coupled with the introductionof faster cars to cause more and more serious traffic jams on roads. Inview of this, greater safety of the vehicles, that is, a braking systemmore reliable in action and more stable in performance than ever, isbeing called for.

With these heavy-duty vehicles, service brakes of the pneumatic typeprevail in which indicated braking pressure from the brake valve isconducted to the relay valve so that compressed air corresponding to theindicated pressure is supplied to the brake actuators via the relayvalve. Since the axle-to-axle load ratio can vary largely according towhether the vehicle is empty or loaded, the air brake system is designedto control the indicated pressure according to the movable load thevehicle carries, or the distance between the upper portion and lowerportion of the spring of the suspension and thereby adequatelyproportion the braking forces applicable to the individual axles,avoiding any unbalanced braking action, such as partial locking of anyaxle alone. In order to obtain the indicated pressure corresponding tothe payload as described above, a conventional brake circuit asschematically shown in FIG. 1 (I) has, between the brake valve a and therelay valve b, a load-sensing proportion valve c which regulates theindicated pressure from the brake valve before the pressure is deliveredto the relay valve. Also included in the circuit is a reservoir d, apower chamber e, and a slack adjuster f. However, installing theload-sensing proportion valve in the line accordingly increases theamount of air retained in the line and, inasmuch as air is acompressible medium itself, the increased air volume in the lineinevitably causes a lag in response of the brake actuators to theinstruction from the brake valve.

SUMMARY OF THE INVENTION

It is a principal object of the present invention to provide a novelrelay valve that combines the features and functions of both aload-sensing proportion valve and a relay valve, so that thetransmission velocity of indicated pressure is increased and quick brakeresponse is ensured.

The relay valve according to the invention, as schematically shown inFIG. 1 (II), combines the loadsensing proportion valve c' and the relayvalve b of FIG. 1 (I) in an integral unit.

In an ordinary air brake circuit, the load-sensing proportion valve c'and the relay valve b are arranged in series, and two hystereses ofoutput pressures of said two valves corresponding to the indicatedpressure are added to each other. Consequently, the output pressure thepower chamber e eventually receives deviates to a considerable degreefrom the value indicated by the brake valve a. The valve according tothe invention, which combines the two existing valves in a unitarystructure, exhibits a hysteresis substantially equal to that of aconventional relay valve.

It is therefore another object of the invention to provide a relay valvewith minimized hysteresis for positive braking action without any timelag in response to the indicated braking pressure.

The relay valve according to this invention, which replaces both theload-sensing proportion valve and the relay valve in an ordinary brakingcircuit, simplifies the air brake system and reduces the assembling costof the system. Still another object of the invention, therefore, is toprovide an air brake circuit which is made more reliable and less costlythan existing ones by the use of the relay valve according to thisinvention.

In order to realize the foregoing objects, the relay valve according tothe invention has the following construction. Like its counterpart inthe conventional relay valves, the valve disk of the valve for openingand closing the brake circuit has an axial center hole as an exhaustpassage and is spring-biased to a normally closed position; it is openedwhen pressed down against the spring. Coaxially with the valve disk,there is installed a control piston unit for opening the valve byactuating the valve disk with the indicated pressure from the brakevalve. The control piston unit comprises a first pressure-sensing pistonfor directly moving the valve disk to open the valve, and a secondpressure-sensing piston engaged with the first piston via a spring, andis equipped with adjusting means for adjusting the position and limitingthe stroke of the second piston depending upon the movable load thevehicle carries.

It is often the case with such large motor vehicles as trucks, buses,and trailer-vehicle which have special body constructions that, for somereason or other, their suspension springs are compressed more than theamount of compression they are normally subjected to when the vehiclecarries a movable load, or are elongated more than when the vehicle isempty. The compression or elongation may be quite outside the normaldeflection range of the suspension springs under no-load and loadedconditions. It is objectionable that such excessive or insufficientdisplacement of the suspension acts on the relay valve.

A further object of the invention is, therefore, to provide a combinedload-sensing proportion and relay valve wherein any excessive orinsufficient deflection of the suspension outside the rangecorresponding to the usual load condition is absorbed by control cammeans associated with linkage so that the service brake pressure isalways adequately controlled correspondingly to the movable load of thevehicle.

The above object is realized by a combined loadsensing proportion andrelay valve of the following construction. In the relay valve of thecharacter described, there is provided a control cam integral with alever which is turnable about a pivot pin, and the cam face consists ofan actuating cam face section for imparting a sliding motion to strokeadjusting means, such as a tappet or thrust rod, in engagement with thecam face to cover the usual deflection range of the suspension springs,and an idle cam face section for imparting no sliding motion to thetappet or thrust rod in the case of any excessive or too smalldeflection. In this way any deflection of the suspension which is toolarge or too small is absorbed and not transmitted to the relay valvebody.

Other objects and advantages of the invention will become apparent fromthe following description of embodiments with reference to theaccompanying drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (I) is a schematic diagram of an air brake system equipped with aconventional load-sensing proportion valve;

FIG. 1 (II) is a schematic diagram of an air brake system using acombined load-sensing proportion and relay valve of the invention;

FIG. 2 is a vertical sectional view of a combined valve embodying theinvention;

FIG. 3 is a graph representing the characteristics of the valve shown inFIG. 2;

FIG. 4 is a vertical sectional view of another embodiment of theinvention;

FIG. 5 is a vertical sectional view of still another embodiment of theinvention;

FIG. 6 is a transverse or radial sectional view taken along line A-A ofFIG. 5;

FIG. 7 is a vertical sectional view of a further embodiment of theinvention;

FIG. 8 is a vertical sectional view of a modification of the embodimentshown in FIG. 7;

FIG. 9 is a fragmentary sectional view showing a modified form ofconnection between first and second pressure-sensing pistons;

FIG. 10 is a vertical sectional view of a combined load-sensingproportion and relay valve equipped with control cam means;

FIG. 11 is an enlarged view of a control cam assembly, showing thepositional relations among its component parts when the associatedsuspension spring is within a normal range of deflection;

FIG. 12 is a view similar to FIG. 11 but when the deflection of thesuspension spring is far less than the normal range;

FIG. 13 is a view similar to FIG. 11 but when the suspending spring hasbeen subjected to an unusual compressive deflection;

FIG. 14 is an enlarged view of a mechanism using a control cam of thetype having a cam slot in which an engaging pin on a rod as thestroke-adjusting means for the second pressure-sensing piston is fittedto actuate the cam;

FIG. 15 is an enlarged sectional view of a mechanism incorporatinganother form of control cam; and

FIG. 16 is a vertical sectional view showing the construction of aload-sensing proportion valve according to the invention.

DESCRIPTION OF THE INVENTION

FIG. 2 schematically shows the construction of a typical relay valveaccording to this invention, as comprising a valve disk 1, a valve body2 accommodating the disk, and a valve bonnet 3 enclosing a controlpiston unit located coaxially with the valve disk 1. The disk has anaxial center hole as an exhaust passage 4. The control piston unitconsists of a first pressure-sensing piston 5, a second pressure-sensingpiston 6, and a spring 7 extended between the two pistons. The firstpressure-sensing piston 5 takes the form of a stepped double pistonconsisting of a large piston part 5' with a diameter A and a smallpiston part 5" with a diameter B. Its pressure-receiving area is,therefore, the annular zone defined between the two diameters A and B.The first piston 5 has on its underside a downward extension 8, which inturn is formed with a valve seat 9 at its lower end adapted to contact avalve seat 10 attached to the upper surface of the valve disk 1. Thisarrangement permits the downward motion of the first piston 5 to betransmitted to the valve disk 1 via the extension 8 and the valve seats9, 10, so as to depress the valve disk 1 and open the valve. On theother hand, the second pressure-sensing piston 6 has the same diameter Bas the small piston part 5", with an indicated pressure applicable onthe entire area having the diameter B, and its downward motion istransmitted to the first piston through the spring 7, until it reaches alimit set by a stroke adjuster consisting of a vertically set rod 11extended slidably through the second piston and a stopper 12 formed atthe lower end of the rod. The rod 11 has at its upper end a horizontallyprotruding pin 13 through which to engage a lever 14, and is moved,together with the stopper 12, upward and downward by the lever. Forexample, if the lever 14 in the position shown is turned clockwise abouta pivot 15, the stopper 12 will be moved from the position a to b andthence to c. The left end of the lever 14 is engaged, for example, withthe underside of a suspension spring (not shown), in such a way that thelever senses a change in the height of the vehicle with the load itcarries and transmits the signal to the stroke adjuster.

With the construction so far described, the combined load-sensingproportion and relay valve according to the invention operates in thefollowing manner.

If it is assumed that the stopper 12 is in the position a as shown inFIG. 2, an indicated pressure Pi from a brake valve (not shown) is ledinto the valve through a port 16 and exerted on the annularpressure-receiving surface of the first pressure-sensing piston 5 andthe pressure-receiving surface of the second pressure-sensing piston 6.The second piston 6 is kept from moving downward by the stopper 12,whereas the first piston 5 is forced downward by the indicated pressurePi applied on its annular pressure-receiving surface. This causes theextension 8 of the first piston to contact and depress the valve seat10, with the result that the valve disk 1 is forced down to open thevalve and admit compressed air supply pressure Pt from an inlet port 17to a chamber 18. While the indicated pressure Pi is exerted on theannular surface of the first piston 5, an output pressure Po acts on theother side of the piston facing the chamber 18. The relation between theindicated pressure Pi and the output pressure Po based on the differencebetween the pressure-receiving areas on the both sides of the firstpiston is represented by a full line O-A in the graph of FIG. 3 whichgives typical characteristic curves of the relay valve according to thepresent invention.

When the stopper 12 of the stroke adjuster has assumed the position b inFIG. 2, the indicated pressure Pi is again exerted on the annularpressure-receiving surface of the first pressure-sensing piston 5 and onthe second pressure-sensing piston 6. In this case the second piston 6transmits the pressure it receives to the first piston 5 via the spring7, until its further downward motion is prevented by the stopper 12.This means that the first piston 5 is subjected not only to theindicated pressure Pi on its annular surface but also to the pressurethat the second piston 6 receives and transmits to the first piston viathe spring 7. Meanwhile, the output pressure Po acts on the other sideof the first piston 5. Thus, the balance between the forces exerted onthe both sides of the first pressure-receiving piston is plotted in FIG.3, wherein the relationship between the indicated pressure Pi and theoutput pressure Po is represented by a full line O-B. The point wherethe second piston is stopped by the stopper 12 is indicated at B in FIG.3.

With a further rise of the indicated pressure Pi; the pressure incrementacts only upon the annular pressurereceiving surface of the firstpressure-sensing piston 5. The increment of the pressure against thesecond pressuresensing piston 6 is borne by the stopper 12 and does notcontribute to an increase in the downward force of the first piston 5.The characteristic is represented by the full line B-C in FIG. 3. Thus,the valve disk 1 is pressed down, opening the valve, and compressed airsupplied at the pressure Pt through the inlet port 17 passes through thespace formed above the valve disk 1 and goes to the power chamber of abrake actuator (not shown) at the output pressure Po via the outlet port19.

The shifting point B can be changed as desired by the positionaladjustment of the stroke adjuster. Further, the ratio of the indicatedpressure Pi to the output pressure Po can also be changed, as indicatedby broken lines and alternate long and short dashes lines in FIG. 3, byvarying the area ratio of the annular pressure-receiving surface of thefirst pressure-receiving piston to the surface of the same piston thatreceives the output pressure, or the area ratio of the sum of thesurfaces of the first and second pistons that are subjected to theindicated pressure Pi to the surface of the first piston that receivesthe output pressure Po.

Should the function of the stroke adjuster to hold the secondpressure-sensing piston 6 be lost on account of a break of the pin 13,lever 14, or pivot 15, the indicated pressure for the second piston 6will be directly transmitted to the first pressure-sensing piston 5through the spring 7. Here the relationship between the indicatedpressure Pi and the output pressure Po will be as indicated by the lineO-B-D in FIG. 3. The valve operation in this case is similar to that ofa conventional relay valve. Thus, despite a failure of the strokeadjuster, the valve according to the invention will effect the necessarybraking action with no danger of insufficient braking force.

When the brake valve has closed and the indicated pressure released, thecontrol piston unit comprising the first and second pressure-sensingpistons is forced upward by a spring 20, with the extension 8 away fromthe valve seat 10. At the same time, the valve disk 1 is lifted, too, bya spring 21 until the valve seat 10 comes into contact with a valve seat22 formed on the valve body to cut off the flow of the supply pressure.The compressed air in the line leading from the outlet port 19 to thebrake actuator is discharged by way of the exhaust passage 4. In thisway the valve action for controlling the actuator for service brake isreleased.

FIG. 4 illustrates another embodiment of the invention, in which theparts like those in the embodiment of FIG. 2 are designated by likereference numerals. In this embodiment, the stroke adjuster consists ofa rod 11 held upright by suitable guide means (not shown) from the lowerend of the valve body 2 and through the valve disk 1 and the firstpressure-sensing piston 5, and a stopper 12 attached to the upper end ofthe rod 11. The stroke adjuster is moved upward and downward (to one ofthe positions a, b, and c) by a lever 14 supported at a pivoted pin 15.In the same manner as in the embodiment of FIG. 1, the lever 14 isconnected to a suitable point of a car body component, such as asuspension spring, the level of which changes with the payload of thevehicle. It will be readily appreciated upon comparison of FIGS. 2 and 4that this embodiment of relay valve is exactly same in construction andoperation as the preceding embodiment except that the rod 11 for movingthe stopper is inserted into the valve body 2 from its lower end insteadof from the upper end.

In FIGS. 5 and 6 is shown another embodiment of the invention. Themovement of the second pressure-sensing piston 6 in this embodiment isrestricted by a lever 25, which is inserted into the valve through thebonnet wall to swing up and down about a pivot 23 secured to anintermediate height of the side wall of the bonnet 3, with the rightportion of the lever 25 as viewed in the figures having a semicircularbifurcation 24. The left end of the lever 25 is usually connected to asuitable point of the lower part of a suspension spring, in such mannerthat a change in the level of the vehicle body is diminished by theleverage and converted to the up-down movement of a pad 26 at the frontof the bifurcation 24, whereby the second pressure-sensing piston 6 iscontrolled according to the height or payload of the vehicle. Thefunctions of other components and the operation of the assembly as arelay valve are the same as those of the two preceding embodiments.

FIG. 7 represents still another embodiment of the invention, in whichthe second pressure-sensing piston 6 is located under the valve disk 1,and the indicated pressure Pi from the brake valve is led via a port 16to the pressure-receiving surface of the first piston 5 and also led viaanother port 27 to the pressure-receiving surface of the second piston6.

The first pressure-sensing piston 5 is connected to the second piston 6via a rod 28, a retainer 29, and spring 7, and the motion of the secondpiston is controlled by a tappet 31 that is actuated by a lever 14,through a cover 30 that encloses the spring 7 and a retainer 29. Ventholes 40, 40', 40" are formed in the bonnet and the cover, the bodyrespectively.

FIG. 8 shows a modification of the embodiment given in FIG. 7. The rod28, retainer 29, and spring 7 used in connecting the bothpressure-sensing pistons in FIG. 7 are replaced by a single spring 34stretched between pins 32, 33 on the inner walls of the both pistons.Thus the modification is featured by a smaller number of componentsrequired.

FIG. 9 shows another modification of the mechanism for connecting thetwo pressure-sensing pistons. A spring 7 is held between the secondpressure-sensing piston 6 and a retainer 36, and a rodlike upwardextension 37 of the second piston is engaged with a lever 14 by a pin13. In addition, the second piston has a rodlike downward extension 38which is slidably inserted into a retainer 36 and is fastened at itsthreaded lower end with a nut 39 to keep the retainer 36 from droppingoff from the downward extension 38.

Shown in FIG. 10 is a combined load-sensing proportion and relay valveequipped with control cam means according to this invention. Generallythe embodiment comprises a valve body 103 accommodating a valve disk 102formed with an axial center hole as exhaust passage 101, and a valvebonnet 107 enclosing a control piston unit which consists of a firstpressure-sensing piston 104, a spring 105, and a second pressure-sensingpiston 106. A bracket 108 is formed on an upper portion of the bonnet107, and a connector 110 pivotally supported at one end by a pin 109 onthe bracket. The other end of the connector 110 is connected by a pin111 to a link 113 formed integrally with a cam 112. The link 113 ispivotally connected by a pin 114 at its left end with a tension rod 115,which in turn is engaged at its lower end (not shown) with a suitablepoint of the lower portion of each suspension spring.

The control cam 112 has an actuating cam face 118 for moving the rod 117upward and downward via a pin 116, and an idle cam face 119 forming anarc with a radius R centered in the pin 111 and contiguous to theactuating cam face 118. The deflection of the suspension spring withinthe usual range under empty and loaded vehicle conditions is covered bythe actuating cam face 118, and the region of excess deflection iscovered by the idle cam face 119.

An indicated braking pressure Pi from the brake valve opened by thefoot-pedal operation enters the pressure chamber 121 of the controlpiston unit through a port 120, and acts on both the firstpressure-sensing piston 104 and the second pressure-sensing piston 106connected to the first piston by the spring 105. The first piston 104 isforced down by the indicated pressure to open the valve disk 102 bymeans of its downward extension 122. Consequently a supply pressure Ptis admitted from an inlet port 123 to the space formed along the valvedisk 102, and leaves an output port 124 as the service braking pressure.The output pressure Po is then supplied to the pressure chamber of thebrake actuator not shown. The second piston 106 too is brought down bythe indicated pressure to depress the first piston 104 via the spring105 but, beyond a certain indicated pressure, its downward movement isrestricted by a stopper 125 attached to the lower end of the rod 117.From then on the first piston 104 takes over the control of the valvedisk 102 to open the valve. With this embodiment, a deflection of eachsuspension spring is transmitted with a reduction in degree via the rod115, link 113, and cam 112, the position of the rod 117 is set by thecam 112, and the valve disk 102 is actuated to open the valve accordingto the payload of the vehicle. In the manner described the service brakepressure is controlled depending upon the movable load.

In the embodiment being described, the lower end of the actuating camface 118 of the cam 112 corresponds to the loaded condition, and theother end of the face that adjoins the idle cam face 119 corresponds tothe empty condition. The actuating cam face 118 is shaped to a suitablecurvature to attain desired characteristics in relation to the indicatedpressure and braking output pressure involved. The idle cam face 119that has the radius R effects no displacement as long as the pin 116 onthe rod 117 remains in contact with the idle cam face 119. The arcuatelength of the idle cam face 119 is so chosen as to correspond to themaximum deflection minus the deflection in the loaded condition of theassociated suspension spring.

As stated, the tip of the actuating cam 118 corresponding to thedeflected position of the associated suspension spring of the vehicle inthe loaded condition, must be positioned to engage the pin 116 when therod 117 is in its lowermost position. In the initial position setting,the inclinaton of the connector 110 is suitably adjusted by turning ascrew 126 and thereby adjusting the position of the pin 111.

FIGS. 11, 12 and 13 are details illustrating the positional relationshipof the control cam 112 and related components in respect of theconnected suspension spring deflected within the normal range, anddeflected excessively and insufficiently, respectively. FIG. 11 showsthe working position the control cam assumes when the vehicle carries amoderate load. The cam-follower pin 116 is in contact with anintermediate point of the actuating cam face 118, and the rod 117 is ina slightly lifted position.

Now if the particular wheel drops in a depression of a road, thesuspension spring will be momentarily relieved of the weight of thevehicle and set free from the compression under load. The springdeflection will be far less than the normal range. As a result, thetension rod 115 is sharply pulled downward, turning the lever 113 andcontrol cam 112 counter-clockwise. This enables the pin 116 to move pastthe actuating cam face 118 onto the idle cam face 119. FIG. 12 shows therelationship of the control cam and the associated parts when thedeflection of the suspension spring is unusually small. Since the idlecam face 119 is arcuately shaped with the radius R, the rod 117 remainslifted in the uppermost position as long as the pin 116 is in contactwith the cam face 119.

Next, it is assumed that the same wheel of the vehicle has rolled over abump on a road. The suspension spring is unusually compressed to absorbthe inertial energy of the body, and the compressive deflection of thespring far exceeds the normal deflection range. Contrary to the caseabove described, the tension rod 115 is sharply pulled up, and the lever113 and control cam 112 turn together clockwise, moving the tip of thecontrol cam away from the pin 116 as shown in FIG. 13. However, becausethe stroke of the rod 117 is limited by the upper stopper 127 that isseated on the upper surface of the bonnet 107, the rod 117 can maintainits lowermost position under normally loaded condition despite theunusual deflection of the suspension spring.

In the embodiment shown in FIGS. 10 through 13, the rod 117 is keptdepressed and there is no possibility of the pin 116 being disengagedfrom the control cam 112. However, in the case where the rod is notalways forced downward, it is impossible to have the pin engaged alwayson its underside with the cam.

FIG. 14 shows the pin 116 engaged with the control cam 112 through aslot 128 formed in the cam plate. The cam slot 128 consists of anactuating cam slot portion 129 and idle cam slot portions 130, 131formed in the front and rear of the actuating slot portion incontinuatioon thereof. The idle cam slot portion 130 corresponds to theexcessive deflection range of the suspension spring, and the other idleslot portion 131 to the range of insufficient deflection. The former isarcuately shaped with radii R₂ and R'₂, and the latter with raddi R₁ andR'₁. Thus, while the cam-follower pin 116 slides within either idle slotportion, the rod 117 will maintain its uppermost or lowermost position,as the case may be, in the same manner as in the immediately precedingembodiment.

FIG. 15 shows the control cam 112 and the related components with a rod117 constantly urged upward, contrary to the arrangement in theembodiment of FIGS. 10 to 13. The tappet 117 is in contact at the upperend of its pin 116 with the control cam 112, and the position of the rod117 is set depending on the position where the control cam 112 turns, oraccording to the deflection of the suspension spring. The control camconsists of the combination of an actuating cam face 132, an idle camface 133 having a radius R₁, and another idle cam face 134 having aradius R₂, said idle cam faces adjoining to the front and rear ends,respectively, of the actuating cam face in the center. The idle cam face133 corresponds to the range of excessive deflection of the suspensionspring, and the other cam face 134 corresponds to the range ofinsufficient deflection.

In the valve of the invention as embodied here, the first and secondpressure-sensing pistons 104, 106 are separated by the valve disk 102located therebetween in a vertical arrangement. The two pistons areconnected together by a rod 135 set upright through the valve disk 102.The indicated pressure is then admitted from upper and lower ports 120₁,120₂ into the respective pressure chambers. The mechanical function ofthe valve disk 102 to open the valve according to the loaded conditionof the vehicle is the same as that which has been described inconnection with FIG. 10, and therefore the explanation is omitted.

FIG. 16 shows an ordinary load-sensing proportion valve incorporatingthe present invention. As shown a control cam 112 is mounted swingablyabout a pin 136 provided on a boss on the right side wall of the valvebody, and a rod 137 is connected to a lug extending from the lower rightpart of the cam. A cam-follower pin 138 fitted in the cam slot enables acontrol rod 142 to slide horizontally. The control cam 112 is formed ofan actuating cam slot portion 139 corresponding to the normal deflectionrange of the suspension spring, and idle cam slot portions 140, 141adjoined to the front and rear ends, respectively, of the actuating camslot portion. The cam slot portion 140, which consists of an arc havingradii R₁ and R'₁, covers the excessive deflection range of thesuspension spring, and the cam slot portion 141, which consists of anarc having radii R₂ and R'₂, covers the range of insufficientdeflection. As long as the pin 138 remains within the cam range ofeither cam slot portion 140 or 141 upon the vertical movement of the rod137, no thrust is given to the control rod 142. Only when the pin 138resides in the actuating cam slot portion 139, or while the suspensionspring is within the normal deflection range, the control rod 142 slidesto adjust the indicated pressure according to the payload that is beingtransmitted to the relay valve connected by piping in series to theload-sensing proportion valve.

As has been described above in the form of preferred embodimentsthereof, the combined load-sensing proportion and relay valve inconformity with the present invention permits braking operation bestsuited to the axle static and dynamic loads of the vehicle regardless ofthe road or driving condition. The invention therefore greatlycontributes to the safe operation of large over-the-road vehicles, suchas trucks, buses, and tractor-trailer systems.

While the invention has been described in its preferred embodiments, itis to be understood that the invention is not limited thereto butnumerous changes and variations may be made without departing from thespirit and scope of the following claims.

What is claimed is:
 1. A pressure control-type relay valve comprising:avalve body having an inlet port and outlet port for compressed fluid andan internal chamber, said body including a closeable passagewayextending from said inlet port and through said chamber to said outletport; a valve disk being interposed in said passageway and cooperatingwith a valve seat formed on said body for opening and closing saidpassageway, said valve disk having an exhaust hole in its axialdirection and being normally biased to close said passageway; a valvebonnet connected to said valve body said bonnet including a controlinlet port; and a control piston unit provided within said bonnet andresponsive to pressure applied through said control inlet port, saidcontrol piston unit including a first pressure-sensing piston forgoverning the valve-open actuation of said valve disk, a secondpressure-sensing piston coaxially connected and resiliently coupled withsaid first pressure-sensing piston and stroke adjusting means forcontrolling the axial movement of said second pressure-sensing pistoncorresponding to the load of a vehicle, said first pressure-sensingpiston being formed as a stepped piston having a large diameter portionand a small diameter portion.
 2. A valve as claimed in claim 1 whereinthe stroke adjusting means comprises a rod extending through the valvebonnet and the second pressure-sensing piston and provided with astopper at its inner end, and a lever engaged at one end with the outerend of the rod and pivotally supported by the valve bonnet.
 3. A valveas claimed in claim 1 wherein the adjusting means comprises a rodextending through the valve member and the exhaust passage of the firstpressure-sensing piston and provided with a stopper at its inner end,and a lever engaged at one end with the outer end of the rod andpivotally supported by the valve bonnet.
 4. A valve as claimed in claim1 wherein the second pressure-sensing piston is located under the valvedisk and seat, the first and second pressure-sensing pistons beingengaged with each other by a stoppered rod which extends through thefirst piston and the exhaust passage of the valve member, a retainersecured to the lower end of the rod, and a spring interposed between theretainer and the second piston, and the stroke adjusting means includinga lever pivotally supported by the lower portion of the valve body, anda tappet adapted to be actuated by the lever.
 5. A valve as claimed inclaim 4 wherein the means for connecting the first and secondpressure-sensing pistons is a spring disposed between and connected tothe both.
 6. A valve as claimed in claim 1 wherein the stroke adjustingmeans comprises an upward extension formed on the secondpressure-sensing piston, a lever engaged with the upper end of theextension and pivotally supported by the valve bonnet, a downwardextension from the second piston, and a stopper engaged with the firstpressure-sensing piston.
 7. A pressure control-type relay valve for anair brake system for a vehicle comprising:a valve body having an inletport and an outlet port for compressed fluid and an internal chamber,said body including a closeable passageway extending from said inletport through said chamber to said outlet port; a valve disk beinginterposed in said passageway and cooperating with a valve seat formedon said body for opening and closing said passageway, said valve diskbeing normally biased to close said passageway and including an exhausthole in its axial direction; a valve bonnet connected to said valve bodyand having a control inlet port, said bonnet also including an internalstepped portion; a control piston unit provided with said bonnet andincludinga. a first pressure-sensing piston responsive to fluid pressureapplied through said control inlet port, said first piston having aportion for bearing on the valve disk for opening said disk, said firstpiston being stepped for cooperating with said stepped portion of saidbonnet, said lower portion being normally resiliently urged away fromsaid valve and limited in movement by the stepped portions of saidbonnet and first piston, b. a second pressure-sensing piston responsiveto fluid pressure applied through said control inlet port, said secondpiston being coaxially connected and resiliently coupled with said firstpressure-sensing piston, and c. stroke adjusting means for controllingthe axial movement of said second pressure-sensing piston correspondingto the load of the vehicle.